In the process of production of steel sheet, the surface of the steel sheet is cleaned for various purposes. For example, cleaning of the steel sheet before plating or painting, removal of oxidized scale by pickling of hot rolled steel sheet (descaling), etc. may be mentioned.
The promotion or increase of the efficiency of such cleaning, the improvement of the cleaning power, etc. are largely achieved by the design of the cleaning solution, but as one further method for assisting cleaning at the time of cleaning, there is the method of applying 20 to 100 kHz ultrasonic waves (Japanese Patent Publication (A) No. 2003-313688, Japanese Patent Publication (A) No. 2000-256886, and Japanese Patent Publication (A) No. 5-125573).
If applying ultrasonic waves in the cleaning solution, a cavitation phenomenon occurs at the surface of the steel sheet, whereby the cleaning effect is promoted. That is, due to the ultrasonic waves, the pressure locally drops in the cleaning solution and becomes lower than the vapor pressure, vapor is generated, or the dissolved gases expand resulting in the rapid formation of small bubbles and cavitation and rapid breakdown, whereby an impact is given while promoting the chemical reaction of the cleaning so as to thereby promote the cleaning effect. Therefore, application of ultrasonic waves is also effective for descaling and pickling of hot rolled steel sheet (Japanese Patent Publication (A) No. 2000-256886).
For the descaling process, a pickling solution comprised of sulfuric acid, hydrochloric acid, nitric acid, fluoric acid, etc. alone or in a mixture of several types is used. To increase the pickling speed of the above pickling solution, the practice has been to increase the acid concentration and raise the pickling temperature etc., but this has minus aspects such as the increase in costs of the chemicals and energy, the orange peel skin of the surface of the steel material after pickling, etc., so there are limits to the improvement of the pickling speed. Therefore, ultrasonic waves are being used together.
However, reduction of the cost of production of steel sheet and improvement of the quality of steel sheet are desired. For cleaning or descaling steel sheet as well, further improvement of the cleaning efficiency and improvement of the cleanliness of the surface of the steel sheet are necessary.
On the other hand, in the fields of semiconductor and electronic devices, as described in Japanese Patent Publication (A) No. 10-172948, the practice has been to clean a wafer for semiconductor while applying 0.8 MHz or more frequency ultrasonic waves (megasonic waves) to the cleaning solution so as to improve its ability to remove foreign matter. Japanese Patent Publication (A) No. 10-172948 discloses a batch-cleaning method of dipping a wafer in a cleaning tank and applying megasonic waves from the bottom of the cleaning tank.
Further, Japanese Patent Publication (A) No. 8-44074 discloses, as a method of efficiently removing a resist in a process of production of a color filter for a liquid crystal display, the method of feeding a curtain-flowing developing solution activated by megasonic waves to the exposed resist.
Compared with 20 to 100 kHz ultrasonic waves (so-called conventional ultrasonic waves), megasonic waves are highly directional, so the surface of the object being cleaned can be efficiently cleaned, the solution molecules are easily activated, and the reaction promoting effect is large.
Therefore, not only in the semiconductor field, Japanese Patent Publication (A) No. 2003-533591 discloses a descaling method using a 500 to 3000 kHz ultrasonic wave source for cleaning rolled copper bars as well.